Radar level gauging (RLG) to measure the level of various filling materials, such as liquids. is an increasingly important method for level gauging in tanks, containers, etc, and many different types of RLG systems are previously known.
One field in which RLG systems are commonly used is for tanks in moving units, such as tanks on marine platforms or on tanker ships, for storage of liquid gas, oil, chemicals etc. These tanks are normally designed as large rectangular blocks, for which the area of the base can be very big, as great as the size of a football pitch with a height of up to 40 m. Since the area of the base of a tank of this type is large, it is of utmost importance that the liquid level can be read with a high degree of accuracy. The large area implies that small changes in the liquid level correspond to relatively large changes in the volume of liquid.
Operation of modern tankers has evolved to also encompass operations outside conventional port terminals, which makes it challenging to meet for example the accuracies of the tank gauging system required according to custody transfer specifications. The encountered situations involve both discharge at offshore located facilities, such as LNG (“Liquefied Natural Gas”) vessels operating at buoys and other gravity based structures (GBS), and ship to ship transfers in open sea, and FPSO (“Floating Production, Storage and Offloading”) offloading to shuttle tankers and lightering ship operations.
In such environment, the vessels will meet sea conditions during the cargo transfer up to its certified limits for safe operation with unlimited tank filling levels.
There is a requirement for custody transfer measurement system (CTMS) to gauge the start and final filling level for the transfer operation with a high degree of accuracy. Further, a frequent number of operations with LNG require the discharging vessel to leave a certain amount of liquid in the tanks for keeping the tanks cold during the ballast voyage to a loading port and/or to provide boil-off gas for subsequent use.
For traditional radar level gauging systems used in controlled environments, e.g. on ships in port, solutions are known to facilitate the interpretation for the operator of the gauged parameter (tank level/ullage), such as low pass filtering or moving average filters. However, simple preset or even adaptive filtering has several limitations. E.g. a setting with high filter factor providing an apparent steady level in open sea environment will react to slowly for use in port operations. Also the filters used currently in level applications are generally designed to handle only smaller variations in liquid movement or designed to remove imperfection in the electronics/microwave circuitry, and there is today no known way of handling situations where the external environment starts to greatly affect the inside tank environment.
Further, in LNG ships, tank gauging systems required to operate accurately down to very low levels, typically down to some cm from the tank bottom, in stable environment, now also are required to provide the same custody transfer compliant accuracy and apparent steady level in open sea environment as they do in port operations under calm conditions.
Some attempts have been made to increase the accuracy of measurements in tanker applications. For example, WO 01/029523, by the same applicant, discloses a special method for reducing the uncertainty of level measurements close to the bottom for LNG tanks. A level measuring system like this also uses low pass filtering of the calculated/corrected level or ullage to provide “smoothing” of the presented and recorded level value for normal small tank liquid surface disturbances caused by the ship encountering light swells, vibrations or pumping to/from the tank. Further, WO 01/029523, also by the same applicant, teaches the use of an absorber to be used at the bottom of the tank to reduce tank bottom reflection. Further, it is known to improve the determinations of very low levels in LNG tanks by use of a bottom located temperature sensor to correlate the measured low level with the actual temperature. When the temperature at this point is significantly higher than the liquid temperature (LNG is typically about −160 degrees Celsius), it means that the tank is “dry” at the measuring point. Such correlation of data for robust determination of empty tank is feasible when the vessel is at a port terminal, but not feasible at open sea, due to the apparent conflict between the periodic (typically 0.1 Hz) behavior of the level as well as the bottom temperature and the normally used smoothing filter.
For the above-discussed reasons, and for both LNG and other applications onboard mobile units, there is therefore a need for an improved RLG system for determining a filling level of a liquid in a tank arranged in a mobile unit in which there are surface waves in the stored liquid due to movement of said mobile unit.